Dark spot corrector



July 13 1948' o. H. sci-IADE 2,445,040

DARK SPOT CORRECTOR Filed May 13, 1943 Sheets-Sheet 1 TLV ATTR/YE Y July '13, 1948. .o. H. SCHADE .2,445,040

DARK SOT CORRECTOR Filed May 13. 1945 5 Sheets-'Sheet 5 l l SIG/VHL RT' OUTPUT 0f I PICK-UP TUBE 10k L' 'w- VERT/cfu '.sHno//ve and +l H WHITE-LEVEL SVG/VIH. l 'l/PPEJJIO/V H7' "VT TUBE aon/voaf I .we/VAL ATM/PUT of HMPL/F/ER ke HORIZONTAL sima/)vs va/vnl. AT TUBE .5'0 CHT/loof RT' TUBE 64 HNODE LEVEL? .we/vm. AT TUBE A -K .9,9 n/vooE l 'W-I l In l] .sy/vc. .www2/ nT ruaE .9e fm/00E "s/a/vm. AT TUBE 13a ale/o \'+L) ATTORNEY be removed by the injection of which are generally known iconoscope type,

, employed Aat the Patented July i3, i94

Otto H. Schade,

of Delaware West Caldwell, N. Radio Corporation of America,

J., assigner to a corporation Application May 13, 1943, seriai'No. 486,766

9 claims. (c1. iis-1.2)

This invention relates to an improvement in vtelevision systems, and more particularly to an improvement in Voi" current modulation of the scanning cathode ray beam, such as, for example, the usual blanking of the beam of electrons.

The undesired spurious signals developed duringthe blanking interval, as well as the undesired dark spot signals, must be removed before transmission of the signals in order that the received picture will appear to have proper tone values without any undesired distortion or improper shading. Furthermore, in the transmission of the picture signals through the various amplifiers at the television transmitter, the direct current component of the signals is lost, with the result that some means must be provided for relating the picture signal and proper voltage level.

' 'Ina television pickup tube of the storage type, there is virtually always some lack of uniformity of response throughout the entire target electrode, and this lack of uniformity of response results in the generation or production of a signal which is generally known as the dark spot signal, and in some instances the variation in signal strength as aresult of dark spot may, 1n fact, exceed the amplitude or intensity of the desired picture signals. A large percentage of the dark spot signal may be removed through the use of back-lighting, but that portion of the dark spot signal which cannot be eliminated byback-lighting must opposing potentials as shading signals. Inconventional television transmitting systems, shading signal generators are normally provided, the generatorsV having means whereby the wave iorm,v as well as the amplitude and polarity of the shading potentials, may be controlled prior to their being combined with the picture signal series.

Furthermore, in a television pickup tubeiof the the scanning cathode ray beam is ,generally blanked or reduced to substantially zero current intensity during the return deflection of the scanning beam, and if no back-lighting is television pick tube, substantiallyno sgnalwillbe generated during vthalliseries to a predetermined .of the brightness of upon the mosaic or -When the scanning Arupted, as during .mosaic electrode. f signal `that is a maintain the white inre, desirable to have a constant amount of light present invention,

i and so shaped that stants that the beam is blanked. If, however, back-lighting is employed, a white Signal Will be generated during the blanking of the scanning cathode ray beam, and the light intensity of the back-lighting source somewhat determines the amplitude or intensity of the generated white signal.

With optimum bias lighting, a white signal of predetermined'value will be generated and this value will remain substantially fixed regardless the optical image projected target electrode if thepickup tube is operated in accordance with the present invention. The white signal that is generated during theblanking interval is caused by photoemission currents from the tube walls to the mosaic or ltarget electrode, these currents being normally balanced by secondary emission from the target electrode during the actual scanning operation.

cathode ray beam is interthe blanking intervals, the production of the secondary electrons at the target electrode is suddenly stopped with the result that the secondary electrons no longer are permitted to ow to the tube walls, but photo-electrons continue to flow from the tube walls to the Since the intensity of the white generated during the blanking ini a function of the number of photoelectrons that flow from the insidewalls of the television pickup tube to the mosaic electrode, the intensity of the white signal will also be a function ofthe amount of light permitted to strike the photo-sensitized walls of the tube. In order to signal more or less uniform terval is then fall on'thetube walls in order that a constant amount of photo-electron emission may flow from the tube walls to the mosaic electrode during the blanking intervals.

Therefore, in accordance with thepresent invention, if the tube-walls are permitted to obtain lightA only from a back-lighting source, this photoemission may be held substantially constant. The therefore, contemplates the use-of a mask positioned in the optical path of the television pickup or camera tube in order to prevent any light from the object area from reaching the tube walls. The mask is so situated light from the object area is permitted to iallonly upon the surface of lthe mosaic electrode, with the result that the photoelectronic emission from the tube kwalls is in respense to 4light incident thereon originating at the black-lighting source only. When a mask is so used, the White level signal may, therefore, be maintained substantially uniform with the result that the undesired dark spot signal may be materially reduced and may be made quite consistent irrespective of the over-all light intensity of the object area, and also irrespective of any changes linthe over-all light intensity of the oblect area.

The white signal generated during the blanking interval is often of appreciable intensity, and for best operation of the television transmitting system, the White signal should be suppressed prior to any appreciable degree of amplication of the picture signals in order' te avoid overloading the picture signal amplier.

In the present invention provision is made Jfor reducing the intensity of the White signals that are generated during the blanking. interval in order that overloading in succeeding stages of the picture signal a'mp'lier vvill not occur.

The present invention also provides means for ingesting shading' 'control potentials in order to remove the undesired variations in picture signal 'strength'tli'at are caused by the 'clark spot response 'and any lack'ofuniformity'of response of the television pickup tube.

Nll/Ieans are also provided in the present invention foiaut'ornati'cally Setting the level of the picture signals after they have been amplified to' a predetermined degree, and this automatic level setting circuit will be `referred to in general as the clamp circuit. Tl'le I'action of the clamp circuit consists Vgenerally in resetting the zero level 'of the video or picture signal duri-rigr horizontal lretrace intervals. In one inode'of operation the intensity of the picture signal 'is brought to fa predetermined fa'djustable value during each horizontal return interval, 'i. e., 'during 'the interval 'between each' successive -l'i'ne "of 'the'pict'ure This particular'voltage Avor vlevel may "be constant, orin 'some instances 'i't may be cyclically 'Variable in vorder to produce va desired verticalshading effect.

The `ieln'CJVa/l Of 'the lln'es'l' Ale'iic'll dark Spot signal campement `11i-ay es accomplished by varying 'the clamp level 'cyclically *during 'each 'lel deectn, callovif-ing for the "ti1t `rexistinlg'inthe natural zro level 'during 'retrace peribets. lSille"theclaip irfctlis'cffotiv lly during :each lior'lzerit'al return inter-val, 'it 'cannot be :used "to alter Afthe Vfsigr'ial intensity 'during vany Aone line yititerval, but ft'l'ie clafnjip circuit may be used ftb alter the picture signal `series in its relation toa predeterminedaiis duri-ng each lineintefrval, and. accordingly the circuit may Ybe iised to eliminate additive signals *of any Vfreluenc'y substantially lower ythan the horizontal 'deflection Vfrequency. iSince 'the horizontal deiie'ction frequency isof lthe-'orli'er offmore 'than`15,000 cycles :per second (assuming presently `existing practice), hum voltages, microphnics and'lw fre- 'quency'dis'tortioniin the f amplifier, as 'well-asthe verticalfdark spctfsignallniaybe removed-by the clamp circuit. The clalnpci'cuitals'o eliminates errors yin l low frequency response" oflthe amplifier,

fano' stabilizes the amplifier insefe'rfas anylt'e'nd- `tney to relaxation oscillations ior bouncing is concerned.

.'By reason'of'thefact-that the'cla'n'lp -cicl'tlis 'enectiveenly during intervals between each suc- 'eeeuiiigfllna it Gannett@ escultor-correctif@any 4 errors occurring 'during each linecr havingafr'e- -quencylo`f voccurrence equal to" or in e'X'cessl off the `line d'eection frequency. 14Beca/useTof 'this-'liu is necessary that the picture 4 substantially zero phase shift or amplitude loss at and above the horizontal deliection frequency.

The invention disclosed herein also includes means for injecting or inserting synchronizing signals that may be used to maintain a television receiver in synchronous operation with the transmitter, the synchronizing signals also being effective, in some instances, to produce the desired return beam blanking in the receiver.

In the present invention provision is also made for obtaining either low or high impedance Vcomposite output voltage variations representative of picture signals, black level signals and synchronizing signals.` A provision is also made whereby a series of `signals may be derived from the systern suitable for application to an oscilloscope for monitoring purposes.

lin addition to a compensation for the undesired vertical dark spot signal through the use of a cyclic variation in the clamp level, the present invention also provides means whereby` the undesired vertical'dark spot signal maybe compensated by modulating the intensity of the signals used to suppress the White signals generated'dur- Ving the blanking interval. Such an arrangement has the effect of altering the tilt of the signals generated during horizontal blanking, and by reason of this alterationA and the `operation of :succeedin'gportions of the system, effective vertical shading may be accomplished.. l

The present .invention also provides a -still `further means fcr compensating forf'the 'undesired vertical dark spot signal :by modulatingthe kcollector electrode 'of a televisionV ,pickup-#tube to thereby vary the potential difference between the inside Wall (the collector electrode) of the television pickup tubetand the .mosaic electrode. A variation inV this potential dilierence affects the number of photo-electrons that are permittedto .iiovv `from the tube' Wall `to .thetmosaic electrode, and also affects the number -fof :secondary electrons and photo-electrons that larepermitte'dito new from tlziefm'osaic jelectrode :to the :collector electrode or ltube Walls. By modulating lthe` :potential of the collector electrode'in a television transmitter tube it is, therefore, 'possible'tofintrovducejin effect, a'desired vertical shading component 'by the elimin'ation'ogf the undesiredverti'cal 'dark .spot vsignal at the television scanning for piekuptube. f

It latherefore, erle@purpose of thefpresent in- 'ventionto provide means'wher'eby the white sign'al generated :during horizontal fblankingintervals may Ibe lm'ainltained substantially uniform zirlrespectiveToftheflightintensity:ofthe objectarea and irrespective iof any `:changes iin the .light :in- -tensity-o'f-theobjectf'area Another purpose "of the Apresent invention' .tresides fin vthe operation Sof ftelevision pickup ftubes Vof the storage f type under, condi-tions whichfmln'i- 'mize dark spot cor'rectionlvoltages,and;Which-operate to maintain the undesired dark spot-signal substantially stable and uniform irrespective r of thelight intensity of the objectif-area in order that automatic level setting' circuits may bel used.

Another purpose lTof v'the lpresel'lt invention'-re- 'sides in'thef'operation of .ic'onoscopes""oresimilar television pickup tubes such that consistent black-levelsiinthe picture-fsignals-maybefmaintained independentA of J gain orl amplificationfad- `justm'ent, together V"with means f'for inserting 'fvertical :shading 1- signals in i combination 1W-ith these circuits. f l

Still iariother-purposef off the f-present'invention yresides infthe operation: cna-television pickuplf A the'ficonos'copei -typlin such Aa manner .that the undesired dark spotr signal will .notchange appreciably with changes 'in the .light intensity of the Aobject' arearthereby to preclude the necessityof altering the intensity of the shading signalswith changes 1n the `light'intensity of the y subject matter being televised.

.Another purpose of the presentinvention resides,` inlthe' provision of meansin a television transmitter' for reducing. the effective intensity of white level signals generated during' blanking of the cathode rayl beamA in a television pickup tube.

A furtherpurpose of the present invention resides in the provision of meansl for Ainserting sha-ding signals atfline deflection frequency for compensatingfor. undesired dark spotsignals in a,horizontaldirection.,A n v v Another purpose of the present inventionresides in the provision of means vfor inserting shading vsignals of vertical or field .deflectionfrequency ,in order toremove undesired darkspot Signals Another purpose of the present invention re#- blanking signal` for reducing the insides in theprovision of means for varying the f automatic level setting means or clamping means cyclically at vertical deflection frequency in order n to compensate for undesired vertical shading effects.

A still further purpose of the present invention resides inthe Iprovision of an amplifier in a picture signal amplifier series with means whereby the potential applied to the control elec'- trode of the amplifier tube may be reset to a predetermined potential value during each interval between each succeeding line element of the picturc.

Another purpose of the present invention resides in the provision of an amplifier tube in a television amplifying system wherein the control electrode of the tube is supplied with picture signals during each Vline interval, and has applied thereto a -predetermined potential between each line interval in order to reset or relate the picture signals to a predeterminedpotential level.

A further purpose. ofthe presentinvention resides in the provision of means whereby the verticalshading signals may be injected into the system by means of the automatic level setting means or clamp circuit, together with means whereby the vertical shading signals may be directly applied to the picture signal series when r the clamp circuit is not in use.

A further purposeof the present inventionre- Another purpose, of the present; invention `rrevsidesintheprovision of means" wherebythe un- 'sides in the provisionY of means whereby the aver- `skilled in the art desired vertical dark spot signal may be eliriii.- nat'edby an appropriate variation in the degree of 'suppression ofthe white signal generated during the horizontal blanking interval.-

, Still another purpose of the present invention resides" in the provision of means for effecting a vcompensation ofthe undesired vertical darkv spot signal by introducing an appropriate tilt'or Vvariation inthe signal strength of the white signal generated during the horizontal blankin'g intervals. f

VA further purpose of the presenty invention resides 'in the provision of means whereby the undesired verticaldark spot signal may be eliminated` by a modulation of the potential of the collector electrode of the 'television pickupv tube. A still further purpose of the present invention resides inthe provision of means whereby the differential potential existing between the mosaic electrode and the collector electrode of the television pickup tube may be cyclicallyvaried in orderto preclude the generation of an undesired .vertical dark spot signal.

A still further purpose of the presentinvention resides in the provision of means whereby both horizontal and vertical synchronizing signals may .be mixed with the picture signals to produce a complete composite series of signals representing the picture signals, the black level signals and the synchronizing signals.

-Various otherpurposes land advantages of the invention* will become more apparent to those upon reading the followingdetailed description, especially when considered in connection with the drawings, wherein t Figuresrla andv lb-show a schematic diagram of a preferred form of the present invention;

Figures 2, 3 and 4-show curvesrepresenting potential variations present-at-various points of the circuit shown in Figures la and lb; and

Figures 5v and 6 represent, respectively; dark spot and lighting conditions of the mosaic. I

' Referring now to the drawings, and particularly to Figure la -thereof,.there is shown therein a television pickup tube II) of the iconoscope or rchargegstorage type having a target ori mosaic electrode |:2 and an electron gun structure I4 to develop a cathode ray scanning beam (not shown) to scan the mosaic. The electron gun structure also includes acontrol electrode lli.l vAssociated withlthe pickup tube is an optical system I8 by meansofwhich an optical image may be projected upon the mosaic or target electrode I2. A mask-|9 is interposed along the optical pathto permit only the'target electrode I2 to be illuminated by light from the object area and to prevent any' light from the lens system from striking the inside walls of the tube I0. Positioned adjacent the television pickup tube I0 is a source of incandescent light 20, which is preferably ener- ,gizedfrom a source of 'direct current applied to the terminal 22, the intensity of the light emitted from the source ZII-beingcontrclled by means of thepotentiometer 2-4. The source of light 20 provides an adjustable amount of back-lighting or wall-lighting for the television pickuptube.`

Y The mosaic or target electrode I2 of thetelef .at ccnductor', and these areappled toa-video signal pre-amplierf28. The intensity 'of thesig- `Vinatlspresent on conductorvZS istincreased bythe preefampiifier 28 and .the ampIiiiedsignalsaaIe .then .applied :to :the :controLelectrode-of a further 4amplifier tube 3.0. Thefpre-ampliiier 28 isshown merely schematically since appropriate :preamplifier circuits are Well .known to .those .skilled in the art, Furthermore, only the .control :elec- .trodeand-anodebf tube :3U fareshown in ordertto simplify the drawings, and theseelements form a suicient representation 'of the amplileritube l'for the purpose of 4the present invention. The cathode circuit of tube 30 is, in fact, somewhat involved, and may actually include an Ielectron discharge @path to 'compensate for the inherent high frequency attenuation introduced bythe load circuit of the television pickup tube, :One .circuit arrangement .for the catlfiode` circuit A-of .tube 30 .is shown and .described, Aifor example, in Schade :Patent No. 2,378,797,1ssued on June 19, .19.45. Various other-'portions ofthe present in- Ventionfare also shown in schematic form, since .the elements `so shown are `Well known 'to V:those skilled 1in theart, and vby representing :these cir-- cuits schematically, the rdescription-of the pres- .ent -invention maybe materially simplied.

QFor blanking the scanning lcathode -ray lbeam generated'in the television cameraor image pickup'tube @fand iconoscope blankingfsignalgen- :erator 'S2-is provided'which genera-tes a voltage :variation such as shown by the curve 34, and these voltage variations are then applic-dto Athe vcontrol electrode I6 oi the televisionpickup tube l| li. 'Since 'the *voltage variations extend in a -negative direction and each impulse occurs during cathode ray beam return intervals, the cathode ray beam -is substantially suppressed or blanked during 1 each horizontal and each "vertical "retrace interval. Y

As explained above, 'by reasonfof the Vpresence of the Ywall-lighting source 2U a signal will 4be generated f-by the ytelevision pickup tube Aduring each blanking-interval, land this signall will extend in a White direction, the 'signal being termed the "white level signal. Since the :intensity of the White level signal is controlledfby the quantity of photo-electrons that continuetoiiiow from lthe tube walls to the mosaic electrode duringthe 'blanking interval, the White level 'signalintensty will alsobe afunotion'of -the light incidentupon the itube walls during such intervals.'Y To maintain the white level ksignal 'substantiallyjconstant it is, therefore, 'des`irable 'to maintain I,the light- 'ingof'fthetube walls substantially constant. `This lis accomplished-in thepresent invention bypro- -vidinglthecmask |19, and when itis positioned, as stated above, -to preclude any light from the .object area'from reaching Vthe .tubewallatfhelightf ing on "the tube walls'may'bemaintainedsnbstantially vuniform since the Vlight thereonforiginates only from Ithe back-lighting source Y211. Even when the maskY |19 is provided, the amplitude 'of thewhitesignal pulse 'may not :remain absolutely constant unless thjepotentlal difference between vthe 'insidewall of the television )pickup tube and the mosaic Avor target is maintained ,substantially constant. 'This constant lpotential can be .obtained `only'vvhen light shading from the source of Ilight 20 'has Abeen adjusted torobtain` a iperffectly uniform dark Signal. However, such a v situation Iis difficult 'to accomplish, 'and valthough the dark spot signal maybe `materially `reduced "by lthe-source of light 2|), some residual isignal wal most 'invariably remains. As a yresult-there "will normally be a certain predetermined ltiltor yvariation #in .the intensity ofthewl'iite level signal, the variation linlintensity ofthe-white levelfsigna'l 8 beingi'cyclicallyirepeated :dor `eacli:vertical:idehem -tiontofltneicathode'rayeheam; l

As twill :be explained dater, "iconoscope" `beam currentsblankingby the blankingsignaLgenerator .32 .is :necessary V.during :vertical and fhorizontal iretracefperiods Lto permit operation :of the automatic level setting Jcircuits :or clamp Vcircuits. Horizontal'blanking'mayg howeven'lbe eliminated, Abut A.wlienfsan automatic level setting circuit is Vused .in :instances whenzno .horizontal @blanking' fis -employedgtbadfstreaking'willresult since the automatic levelisetting` circultieifectively `removes the low frequency picture signal component. .fit .is desirable,thereforathat bothhorizontaiand .ver- Y"tical :'blankingi be employed rand', ifurthermore, it `is :desirable 'that :especially l'the :base .line zbetween zblankingfimpulses have -faconstant .level 1in=order to eliminate undesired beamicurrent.modulation ortransients.

The "iconoscope"'ibla1iking signal .generator represented .schematically at 32 iin fthe drawing, therefore, nsupplies "both V.the I'horizontal and -vertical-tblanking signal 'impulseslasta combinedcom- -posite seriesfqffsig'nals.. 'The compositeseries of blanfking' signals may Phe igenerated in Ithe proper wavejformandin acompositeseriesby employing .an limpulse mixing circuit such-as shown and'describedsiiniischade Ratent-fNo. 2,`363, 8U9,issued on .'Novenrberr28, 1944. *When separate y:impulses of :horizontal fand ivertical deflectionY ffrequency are applied to theimpulse mixing-circuit, the fim- -pulses arefconibined andare lalso made @rectangu- -larin'wave "form.. inasmuch as `thefimpulse mixing circuitlls ,'clearlyishown Pand described inthe above :mentioned application, Y.it 'is not believed that it need be specifically shown'andfdescribed herein.

:For simplicity; .the operation of the present mkventi'on P.will irst be idescribedon the assumption :that the white `:levelhs'ignal remains substantially lcoiistant Land also son the vassumption that no dark spot signal is present in the picture signal series vgenerated andv supplied by uthe television .pielrupftube `Furthermore,it will also be assumed that the automa-tic ilevel `setting circuit or clamp circuit `isnotemployed. Under tli'eseassurned circumstances, `thel series of V'curves shown in "Figure i2 are applicable. 'il'n 'thesecurves' 'tWo loperatingcondi-tionsfhavebeen shown, one condition being for a dark or unillumin-atedmosaic, andthefother ucondition being 4when an optical imageiisproje'cted on the mosaic.

'In order to assist -in' relating lthe ,curves shown in`Figures2, v-3 landsiito the circuit shown ,in Fig- Auresrl-a and flu, ^the curve identifying V'letters vhave ibeen Iapp'liefilto"the circuit diagram'at the appropriate points. Curves Afand 'B showninfFigure V2, therefore, represent voltage variations .available at the output .conductor 2'6` of the itelevision 'pickup tube A'-Iftl V'under conditions of La dark mosaic land of 'anv illuminated mosaic, respectively, It 'will 'be noticed that lin vthe curve shown Aat A in Figure 2, the white `level-signal lremains constant land, furthermore, substantially Zero signal 4is lgenerated between each lblariking =`int erval,V findicating `a darkmosaic and the lack 'ojf a :dark Spot nsignalf "In curve B, the -Whlteleyel sign-al .is also 4constant but `interposed between each blan'king interval `are series :of `picture signals, each vrepresenting a line-element "of'th'e picture. 'It-will also A'be obsfervedf-in'curve Bof' Figure 2 thatthe white `level signa1 is substantially equal to or .exceeds the maximum whiteplcture or'video signal.

yThe signalsgrepresenteizl -byfcurves ;A ,and 'B of Figure '2*are then applied, as statedabove, to the video signal; pre-ampliiier 28, where their intensity is increased, and after amplification they are then applied, in reversed polarity, to the control electrode of another amplifier tube 3&3. As stated above, only aportion of this tube is shown for the sake of simplicity, and the anode of the tube is maintained at a positive potential by means of a load circuit includinga series peaking coil 32 and vparallel resistance 34, a further inductance member or shunt peaking coil. 3b `and a load impedance 38. This load circuit is connected 'between the anode and terminal lill to which a positive potential isl applied. The amplified picture signals are then applied to a further video amplifier 42 by way of coupling condenser llt.

For reducing the intensity of the white level signal, a blanking signaler impulse series is in.. serted between the-amplifier tube 3l) and the amplifier 42. This blanking impulse series is pro-` vided by the blanking signal generator lili which generates' a wave form vsuch as shown at 48. :These same blanking impulses that are used to reduce the intensity of thewhite level signals are l'also represented by curve C of Figure 2, and when 'these blanking or white level signal suppression impulses are superimposed upon the picture sig- 'nal series present in the anode circuit of tube 3G, the effective amplitude of the white level signals :may be brought to substantially zero (i. e., to a level representative substantially of black in the picture).

The picture or video signals present at the confductor 25 have a polarity such that whitev (e.l g., :a high light in the scene being televised) is in a megative direction, and if a single or an odd nurn- 'ber ci amplifier stages are includedy in the preamplifier 28, the picture or video signals in the 'anode circuit of tube Sli will also have a polarity 'such that white extends in a negative direction, 'as indicated in the curves Aand B of Figure 2. After the bl-anking impulses are combined with `the picture signals to suppress the white level signals, a series of signlalsrwill then result corresponding to vcurves D and E Vof rFigure `2. Curve D is, therefore, the combination of the curve A and the curve C, while curve E represents a coinbination of curves B and C.

It will be observed that the duration oi the blanking or `suppression pulse exceeds the ,telelvision pickupv tube blan'king interval, with the result that a short positive or bllacker-than-black signal will result. Curve D, therefore, represents the potential variations at the input of video amplifier i2 under conditions of a dark mosaic,

While curve E represents the potential variations present at the input of amplifier 42 for the condition of an optical image on the mosaic of the television pickup tube.

After the signals are lampliiied by the amplifier 42 they are then supplied to a further amplier tube 5d having at least a cathode, a control electrede and an anode. The anode is connected to a terminal 52 to which is applied a positive potential, while the cathode `of tube 50 is connected to ground by Imeans of a cathode loadresistance or impedance 54. Naturally, ampliiier picture or video signal output may be derived from the anodeof tube lill orl if, as indicated in the drawing, a low impedance output is desired, then the picture or video signal output may be derived from the cathode Aof tube 5). This connection is frequently desirable sincein some instances it maybe necessary to transmit the picture signals QYQI'. .2l OlXal lbleft a .further Vdeo amplifier or so-called line amplifier. The coaxial cable is,A therefore, represented at 55 and this cable terminates at a volume control or potentiometer 58 having a movable contact Bil.

As stated above, it is assumed in this description of the operation of the present invention that no dark spot signalis present at the output oi the television pickup tube, and consequently no shading is necessary. Accordingly, a description of the injection of the shading control potentials will-not at present be given. f y

Picture or video signals having an intensity. determined by the position of the movable contact 60 along the volume or gain control 58 are then applied, by conductor 6l, to a further video-or picture signal amplifier 62 (see Figure 1b), Ythe output of which supplies signals to the control electrode of a further amplier tubell,l This amplier tube includes at least a cathode, a control electrode and ananode, and the cathode is connected to ground through a series connected inductance 66 and resistance 68. The inductance has connected in parallel therewith a resistance 10 and condenser 12, while a further condenser I4 is connected in parallel with the resistance 68. An adjustable condenser 16 is connected between the cathode and ground or a point of=fixed potential. This network provides a transient compensating network, and when thecondenser 'lllI is properly adjusted the effects of undesired transient conditions in the fampliiier may be nullifled. Y l Y Y The anode of tube B4 is maintained positive with respect t0 its associ-ated cathode by connection to positive terminal 18, the 4connection including series peaking coil 80, parallel-,resistor B2, series inductance 84 (shunt peaking)V and load impedance 86.' Voltage variations present at the anode of tube 64 will then have a wave forni simi-.- lar to the curves D and E of Figure 2, except that the polarity will be reversed. Accordingly, White in the picture will extend in a positive direction. For assuring that the signals present at the anode of tube 64 during beam return time inter,- vals have an intensity at least equal to or extending beyond black in the picture, means is-provided for injecting blanking impulses into the signal series at the anode of tube 64.y yWhen'these blanking impulses are then combined with the picture signals present -at this point, the intensity of the signals occuring between adjacent Yline picture signals will always be at least equal to -black'and extend in a blacker-than-black direction regardless of the position of slider 6U on the gain controlV potentiometer 58 (Figure lai.,

For supplying these blanking impulses, a kblanking signal generator 88-is provided which generates a wave form such as shown at 99 inFigure 1b. This same wave form is also represented by curve F in Figure 2, and when these impulses are mixed with the picture signals present at ithe anode of tube 64, a resultant series of signals corresponding to the ycurves G and H of Figurev 2 ,will'be produced. A

It will be noticed by comparing curveslGand I-I with curves D and E, respectively, that the signals present during the blanking interval arey caused to havey an intensity extending in the blackerthan-black region (black level is indicated bythe dotted line in curves G and H).

y*As stated above, it is assumed that the clamp circuit is not in use and, accordingly, the switch S2 will be assumed to be in the -clamp,offposi tion, i. e., in engagementy with contact C`2 I. .Under these circumstances, voltage variation such il as represented by curves G and H in `Figure 2 will then be supplied to t-he control electrode of a further amplifier tube 92 by way of coupling condenser 94.

The tube 92 includes at least a cathode, a controlelectrode and an anode, and the cathode is connected to ground by way of cathode resistance 96. This resistance has a condenser 98 connected in parallel therewith when the switch 99 is in the closed position. However, when the switch is placed in the open position the condenser is not included in the circuit. This switch provides a gamma control of the picture signals and by-positioning the switch in the closed or Athe open position, the black level clipper tube 92er amplier tube may be made to have a linear amplificationor may be caused to amplify according to a square law. When the condenser 98 is included `in parallel with the resistance 95, the characteristics of the amplifier tubeare accordin-g to `a square law, Whereas if the condenser 98 is not included in the circuit, the amplification is; substantially linear. When operated as a linear amplifier, the iconoscope gamma is pre served, which is desirable for optimum or good lighting conditions. If, however, the lighting conditions are poor, then the condenser 98 should be included in parallel with the resistance 95 to causeA the tubel 92 to have a characteristic such that a square law amplitude distortion with increased gain for signals representative'oi white may result to correct for the change in response of the iconoscope under poor lighting conditions and to produce a substantially linearl signal versus light characteristic c r uni-ty gamma, which f;

is psychologically desirable under poor lighting conditions.

'Iher anode of tube 92 is maintained-positive withrespect' to its cathode by means of peaking coil` |`|J2- and parallel resistance |04, as well as inductance |09 and loadA impedance or resistance These elements are connected between the anodeand terminal l to which a source of positivepotential is applied.f

The tube 92, assuming the switch S2 to be in the clampoff (02|) position, has its control electrode maintained at a xedaverage potential with respect to the cathode. For maintaining this predetermined bias, a black level control potentiometer ||2 is provided, the resistance element of the potentiometer being connected between negative terminal |.|4 and a point of xedpotential or ground. A mov-able contact I I6 of the potentiometer is connected to the control elecltrode of tube 92 by way of grid resistor H8. In

order to lter the potential applied to the terminal H4, a condenser |29 is provided which is connected between the movable contact and ground. By an adjustment ofthe movable contact `H6 along the potentiometer resistance ||-2, it ispossible to vary the average potential (i. e., thedirect potential) of the control, electrode of tube 92 with respect to its cathode with theresult that the clipping level of the tubegmaythereby'be controlled.

In curves G and I-I of Figure 2, dotted lines are shown,corresponding'substantially to black in the picture signahand this, will be assumed tobe the clipping level oftube .9,2 fora predetermined positiongof the movable contact llt. Under these conditions any signal having a negative polarity exten-ding beyond the clipping level will bias the tube 92 to cut-off, with the result that no current will pass therethrough. Accordingly, a picture signal series will be present at the anodevof tube 12 92 corresponding substantially to the cur-ves J and K` of Figure 2.

Curve J of Figure 2, as indicated, is the voltage variation present when the mosaic electrode |2 is non-illuminated, whereas curveV K represents the voltage variations under `the conditions when an optical image is projected on the mosaic electrode |2. Itiwill also be observed, particularly from curve K of Figure 2, that the signals representative of white in the picture extend in a negative direction at the anode of tube 92. The voltage variations at this point in the circuit include only the picture signals with the potential level of the voltage variations reduced to a value corresponding to black in the picture for those intervals between adjacent picture lines or line elements.

At this point `the synchronizing signal series is injected into the circuit in Aorder that a composite series of picture, black level and synchronizing signals may be, produced.

For the, injection .of the synchronizing signals tube |22 is included, `this tube comprising two separate triode elements. A series of impulses of yvertical deflection frequency having a Wave form W. similar to that'indicated at |26 is supplied to control electrode |24, while impulses of horizontal deflection frequency having a Wave form such as indicated at t28-are applied to the control electrode |30'. By reasonof the operation of tube |22, a composite series of impulses is' then present in the output conductor |32 representing a composite of both horizontal vand vertical synchronizing impulses. Thes'eimpulses as present at conductor |-32extend in. a positive direction, and are added to the picture-signal series present at the anode of tube 92 rby parallel insertion through condenser |34 and resistance |36. The duration of thehorizontal driving impulsesrepresented by curve |28 is preferably 30% to 40 shorter than the blanking impulses supplied by the generator 88,. so4 that i the lhorizon-tal synchronizing impulses do not ex- Ytend throughout the entireblanking interval.

The vertical synchronizing impulses have a duration corresponding to 11/2 to 2 line intervals, this durationA being suiiicient to maintain the receiver. in synchronous operation. The composite series of synchronizing signals as provided by tube |22 is not in accordance with the present R. M. A. standard when such driving impulses are applied to the control electrodes of tube 22; however, by application of proper driving impulses, a series of composite synchronizing signals could be generated. corresponding tothe. prescribed R. M. A. standard series.` The operation of tube |22 will not `be explained' in detail since its operation is clearly shown and described in the above mentioned Schade Patent No. 2,363,809.

The synchronizing impulses supplied by tube |22 are represented by curve L in Figure 2, and when these are combined with the voltage variations present at the anode of tube92, a composite series of signals will be produced similar to that represented by curves M and N of Figure 2. In the curves represented at M and N it willV be noticedthat the signals representative of white extend ina negative direction, .-and the synchronizing'signals extend in a positive direction. Signals of =this polarity are present inthe anode cir- Y form such as vindicated by curve C in Figure 3 (seealso curve 48 in Figure 1a). These impulses, as previously stated, extend in a positive direction, and when combined with the signal present at the anode of tube30 produce a signal series such as represented by curve E of Figure 3. The combined voltage variationsare then applied to the picture signal amplier 42.

By inspecting curve E of Figure 3, `it may be seen that only the white level signal isV affected, the picture or video signals remaining substantially the same as generated. It will also be observed that the white level suppression signal represented by curve C has a width greater than the time duration of the white level signal in order to eliminate any undesired transients that may occur at either extreme end of each line deilection. v v

The picture signals represented by the curve E in Figure 3 as applied to the video amplier 42 are also present at the cathode of tube 5U and at the volume control potentiometer 58, except that the polarity of the signals at these points is reversed from that shown in curve E of 'Figure 3.

At this point in the circuit horizontal shading potentials are mixed with the picture signals in order completely to cancel the undesired horizontal dark spot signal component present in the picture signal series. For this purpose a horizontal shading signal generator I 68 is provided to which is applied an impulse series of horizontal deflection frequency (not shown). The horizontal shading signal generator includes ya circuit arrangement whereby voltage variations of both sawtooth Wave form and parabolic wave form in a particular, or either polarity, may be generated with provisions whereby the voltage variations may be conned to one or another portion of the line element. Circuit arrangements for generating horizontal shading control potentials are well known to those skilled in the art, and an example of such an arrangement is to be found in Schade Patent No. 2,312,054, issued on Feb. 23, 1943, or in Seeley Patent No. 2,271,876. granted on February 3, 1942.

For the purposes of the present invention, it is not material what type shading signal generator is employed, since the present invention does not relate to any particular shading signal generating means. Accordingly, and in order to avoid complexities, the shading signal generator is represented schematically and supplies separate shading components, one having a sawtooth wave form as applied to conductor |62 of Figure la, the other having a parabolic wave form ,1

as applied to conductor |64 of Figure 1a. Associated with these conductors are potentiometers |66 and |68, respectively. The movable contacts of these potentiometers are connected to conductor |16 by Way of isolating resistors |61 and |69. By varying the position of the movable contacts along potentiometer resistances |66 and |68, the intensity of the sawtooth and parabolic voltage variations may be individually controlled. The conductor |1Il is then connected to the high potential end of the volume control potentiometer resistance 58.

There is shown by curve P of Figure 3 the shading signals necessary to remove the undesired horizontal dark spot signal from the picture signal series. This shading control potential includes components of both sawtooth and parabolic wave form, and for simplicity these wave forms are shown individually although naturally in actual practice a combination of the two would, in fact, be used; When the voltage variations such as shown by curve P of Figure 3 are combinedv with vthe picture signal series such as represented by curve E of Figure 3 (but with reversed polarity) a signal or voltage variation will then be produced similar to that represented by curve R of Figure 3; This voltage variation is present at the input of the video or picture signal amplier 62, shown in Figure 1221 By inspecting curve R of Figure `3, it will be observed that the undesired vhorizontal dark spot signal has-now been completely removed, with the result that for each line element vpicture signals'representative of gray (for example) in the picture have a fixed and definite value with no slope or deviation. The same is true for all other picture signals representing a predetermined light value within the same line element. It will, however, be observed :that for each succeeding line element the averagevalue of the picture or video signals does not remain xed, and Ythis deviation of thezaverage value is a result of the undesired vertical dark spot signal component.

Thepresent invention also includes the use of a vertical shading signal generator |12, which mayl be similar to the horizontal shading signal generator |60 except that itis supplied-With `impulses of vertical deilection frequency.` Voltage variations oi"y sawtooth and parabolic wave form are generated by the vertical shading signal generator, and theseivoltage rvar iations are present at conductors |14 and |16.- They are applied, respectively, to potentiometers |18 andV |80 in order that their intensity may be controlled, and the combined vertical shading component is then present onconductor I 82. Isolating resistors |79 and IBI are employed between the potentiometers and conductor |82. ,This conductor is connectedto the movable arm of switch S-I in order that the vertical shading `signals may be applied to one or another pointin the television system. ,y

As shown inthe drawing, switch S-l is inthe clamp off, position (contact CI'I) however, when-a clamping action is used, as `will be described in this ,assumed operation, the vertical shading signals cannot` beY injected along: with theY horizontalshading signals since the clamping action of the automatic level settingcircuit would immediately remove the vertical-shading component by the clamping action unless the vertical shading signal component were notched or slotted at horizontal deilection frequency phasedwiththe white level signal in ordernot to alter the intensity of the white level signal. If both the picture and the white level signals were altered in intensity by the vertical shading signal generator, this alteration would be removed by the clamp circuit, (but if onlythe one or vthe other signal is a1tered,rthen @verticalv shading effect can be produced. Inasmuch as notching or slotting of the vertical shadingsignal requires considerable complication of circuits, vertical shading has not in general been accomplished in this manner. Accordingly, the `undesired variation in picture signal strength because of the vertical component of the dark spot signal will-be removed by a manner later to be described, and for this purpose it willbe assumed that the switch S-I is in the clamp on position, i. e., cooperating with contact CI 2. f Likewise, as indicated inthe drawing, since switch S--I and switch S-Z (see Figure 1b) Vare ganged together, the switch S-Z Will also be in the clamp on position, i. e., cooperating with contact C22. f

have picture signals corresponding to the curve shown at R in Figure 3 present at the input of the video or picture signal amplier 62. These signals are then amplified, and are, in turn, applied to the control electrode of amplifier tube 64, and if an odd number of stages oi amplification are included in the ampliiier 62, then a signal variation vsimilar to and having the same polarity as that represented by curve R of Figure 3 will be present at the anode of tube 94.

At this point in the circuit blanking impulses are combined with the signal series and, as previously described, the blanking impulses are obtained from theblanking pulse generator 08 and have a wave form such as indicated by curve F of Figure 3 (see also curve 90 of Figure lb).

We noW These, when combined with the signals repre- Y sented by curve R, produce a wave form such as indicated by the curve Hof Figure 3. After the blanking impulses have been combined with the signal series, it may be seen that the intensity of the signals during the return time is at least equal to or greater than black in the picture signals. These picture signals are then applied to the control electrode of tube 92 by Way of coupling condenser |06, contact C22 and switch S--2. Tube 92 performs a clipping action since the tube is biased to cut-off for signals extending in an excessive negative direction, and the potential applied to the control electrode of tube 92 is such that the voltage variations represented by curve H are clipped at the level indicated by the dotted line. It will be observed that this level changes for each line element, and this change of clipping 'level is broughtaboutby reason of the action of the clamp or automatic level setting circuit which will now be described.

The clamp circuit includes four uni-directional current conducting paths |88, |90, |92 and |94. These paths may preferably be simple electron diodes, and in actual practice may constitute two separate duo-diodes such as, for example, tube type 6H6. In order that they may be more clearly represented they are shown in Figure 1b as separate diodes each having a cathode and an anode. The cathodes of diodes |88 and |90 are connected together, as are also the anodes, of diodes |92 and |94. Each of the diode elements may be looked upon as separate arms of a bridge circuit, the entire combination op erating as an electron switch connected across one diagonal of the bridge. For causing operation of the electron switch, potential variations of opposite polarity and of similar wave form are applied to' the cathodes of diodes |88 and |90 and to the anodes of diodes |92 and |94. These voltage variations are derived from tube 200 which includes a cathode, a control electrode and an anode. The cathode of tube 200 is connected to ground, whereas the control electrode has applied thereto potential variations of horizontal deection frequency, the potential variations extending in a positive directionand having a duration approximately corresponding to the return time of the scanning beam in the cathode ray image pickup or scanning tube. age Variations, represented by curve in Figure lb, are applied to the control `electrode of tube 200 by way of -coupling condenser 202, and the control electrode vis connected to ground by the usual grid resistor 204.

. The anode of tube 200 is maintained positive with respect to its associated cathode by the primary winding 206 of transformer 2|0, one end of the primary winding being connected to the These volt- 18 anode of tube 200 and the other to positive terminal 2|2 by way of resistance 2|4. The voltage variations present at the anode of tube 200 are such as those represented by curve 2|4 of Figure 1b, and these voltage variations induce a potential variation in the secondary winding 2I0 of transformer 2I0, having a similar wave form.

One end of the secondary winding '2|6 is connected to the cathodes of diodes |88 and |90, whereas the other terminal of the secondary 2|0 of transformer 2|0 is connected to the anodes of |92 and |94. K

The anode of diode |88 and the ycathode of diode |92 are connected together, and in turn are connected to the clamp on terminal of switch S-i. (contact C22) by Way of conductor 2|8. The anode of diode |90 and the cathode of diode |94 are connected together, and in turn are connected to the movable contact ||6 of potentiometer ||2 by way of resistance 220. Since arnegative potential is applied to the terminal ||4 for biasing the control electrode of tube 92 When the switch S--2 is in a clamp off (contact C2 I) position, this same average negative potential will also be applied to the anode of diode |90 and to the cathode of diode |94. y

When the switch S-2 is in the clamp on (contact C22) position, the biasing of the control electrode of tube 92 is effective insofar as the position of movable contact ||5 is concerned only When a conducting path is establishedby the four diodes of theclamp circuit. This conducting path is present only for a short interval and for a time coinciding substantially With `the return deection of the scanning cathode ray beam.

In the operation of the clamp circuit, when voltage variations or impulses of horizontal deflection frequency are applied to the control electrode of tube 200, voltage variations such asrepresented' by curvesv A222 and 224 Will be present atthe output terminals of the secondary 216 of transformer 2|0. These voltage variations are then applied respectively to the cathodes of diodes and |790, and to the anodes of diodes |92 and |94. When these impulses are applied to these tubes, they are rendered conductive inasmuch as negative impulses are applied tothe cathodes of diodes |08 and |90 and positive impulses are applied to the anodes of diodes |92 and |94. During this short` conducting interval the charge present on condenser |06 is re-established to correspond to the position of the movabley contact H0 alo-ng the potentiometer ||2. l

The action of this clamp circuit or automatic level setting circuit consists in re-setting the zero level of the video or picture signal to a desired value during each horizontal retrace period. As so far described, this value would be constant as determined by the position ofthe contact H0 along potentiometer ||2. However, this value may be cyclically varied in synchronism with the vertical deflection of the cathode ray beam in order to effect a vertical shading of the picture signal series. Accordingly, a shading component may be superimposed upon the normal xed potential determined by the position of the contact H6 along the potentiometer H2 to remove the undesired vertical component of the dark spot signal. The action of the clamp circuit will, therefore, eliminate additive signals of substantially lower frequency than one hori- Zontal cycle, i. e., hum voltages, microphonics, and low frequency distortion in the amplifier. 'it also eliminates errors in low frequency re- 19 sponse and, in addition, its action stabilizes the amplifier tendency toward relaxation oscillations or bouncing.

The clamp circuit as described, in view of the fact that it is effective only during each horizontal retrace interval, cannot correct errors occurring between each horizontal return interval, since the clamp circuit is ineffective during each individual picture line interval. The amplier, therefore, should have substantially zero phase shift or amplitude loss for all frequencies equal to or greater than the horizontal scanning frequency.

The provision of the'transformer 2 l0 is for the purpose of providing a floating triggering potential for the diodes of the clamp circuit and, furthermore, the transformer delays the pulse front sothat the diode bridge conducts approximately at the center of the retrace period, i. e., at the center of the interval occupied by the white level signal. The diode bridge circuit which constitutes the clamp circuit must be balanced to prevent insertion of the clamping pulse itself into the picture signal series. This balance requires equal capacitances as well as equal resistances. The bridge capacitances (i. e., the capacitances of the diodes) become unimportant when the conductances of the bridge are low, which in turn requiresa reasonably high pulse current in order to produce the necessary potential variations.

It will be further represented at 222 and 224 of Figure 1b is somewhat rounded or sinusoidal rather than purely rectangular, and this sinusoidal condition is preferable in view of the fact that fewer high frequency components are present in the control potential series. The entire clamp circuit including the diode bridge or rectier circuit produces its own cut-01T potential, and this bias must be higher than the largest picture signal potentialin order to maintain the bridge non-conductive between triggering impulses.

In order to remove the undesired vertical dark spot signal component present in curve R of Figure 3, the clipping level or clamp level is cyclically varied at vertical deflection frequency. This is accomplished by supplying the vertical shading signalby way of switch S-l and contact CIZ to the anode `of diode |90 and the cathode of diode |94. i The clamp on Contact C|2 of switch S-l is, therefore, connected to ground by way of potentiometer resistance 228 and the movable contact 230 of the potentiometer is connected to these elements of diodes I9!) and iSd by way of conductor 23| and coupling condenser 232. The resistance 220, referred to above, is included in the circuit in order to decouple the shading signal component fromthe black level setting of the potentiometer ||2. The average potential applied to the anode of diode |90 and the cathode of diode |94 is, therefore, amplitude modulated in accordance with the vertical shading component and a portion of this amplitude modulation is represented by the solid line of curve S of Figure 3.

Inasmuch as the clamp circuit is effective only during the retrace interval, as explained above, the effective potential insofar as the coupling condenser I 86 is concerned is such as that represented by the dash-dot line of curve S of Figure 3. Since the clamp circuit places a `different base charge on coupling condenser |86 during each line return time interval, as determined by the vertical shading component, the clipping level is cyclically varied as indicated by the dotobserved that the wave form ted line'in curve H of Figure 3, and when the signalis clipped by tube 92 along the dotted line shown in curve I-I, a resultant voltage variation is produced at the anode of tube 92 such as that represented by the curve K of Figure 3. By inspecting this curve, it maybe noticed that the undesired vertical dark Aspot signal has been completely removed and the signals are related to a predetermined potential level with the result that signals representative of a predetermined light value have a predetermined and xed intensity. Also, when the position of the movable contact H6 is properly adjusted, the signal strength of the signal series during the blanking interval has an intensity corresponding to black in the picture.

It will further be observed that signals representing those portions of the picture shown in Figure 6 that. are black have a uniform level, as do also picture signals representative of gray or white. At this point in the circuit all of the undesired horizontal and vertical dark spot signal` has been completely removed, and furthermore, all undesired signals occurring between line elements have been removed and the signals during these intervals have been brought to a level corresponding to black in the picture.

In Figure 1a of the drawing it will be noticed that the movable arm of the gain control 58 is mechanically connectedto the movable arm of the vertical shading signal potentiometer 228. Suchan interaction is desirable so that the intensity of the vertical shading signals as applied to theA clamp circuit may be varied in accordancewith the selected position of the gain control. Naturally if the over-all gain of the amplier were increased the strength of the shading control signals should also be increased to maintain the same degree of shading. By couplingthe two controls, it therefore eliminates the necessity of readjusting the shading potentiometer each time the gain or Volume is altered.

It is then only necessary to introduce the synchronizing pulse series .as represented by curve L in Figure 3, and this is accomplished in the anode circuit of tube 92 by the action of tube |22, as explained above. Accordingly, the series o=f signals present at the control electrode of tube It is `such asA thatrepresented by the curve N in Figure 3, and this voltage variation includes synchronizing signals, black level signals, and picture signals properly related to a common base level in order that these signals may then be used to produce a picture on a .television receiving tube or to modulate a radio frequency carrier for transmission, as explained above.

t may be seen from the above, therefore, that undesired dark spot signals maybe removed or cancelled in a television transmitting system, and the picture signals may be related to a predetermined desiredr blacklevel so that the transmitted picture signals will be properly representative of the light values of the optical image. When the television pickup. tube is so operated that the mask i9 prevents any light from the lens system from reaching the tube Walls, the White level signal will remain substantially constant. By adjusting the position and intensity of the backlighting source 20, the intensity of the produced dark spot signal may be materially reduced and the remaining dark spot signal may be removed by inserting horizontal shading signals at a point in the system so that no undesired deviation in picture signal strength will be present for any particular line element. The undesired vertical 21 dark `spot signal component may be removed by altering the clipping level through the use of a clamp circuit which operates during the white level signal interval (return time interval) to vary the clipping level in accordance with the required vertical shading component. The resultant picture signal series is, therefore, free from all undesired dark spot signal potentials and may be used to modulate a radio frequency carrier prior to transmission of the signals to remotely located television receivers. On the screen of the receiver a picture image will be reproduced Which is truly representative of the proper light values of the original optical image.

Inasmuch as the white level signal does not vary in accordance with variations in the average light intensity of the masked image projected on the mosaic, it is not necessary to alter the shading signals in either intensity or wave form when the average light intensity of the mosaic electrode is changed, as, for example, during a change in the scene. I-Ieretofore, whenever the average light intensity on the mosaic was changed due to panning or movement of the iconoscope camera or pickup tube, or due t-o a change in the light value of the objectl projected on the mosaic electrode, it has been necessary to alter the shading components since the dark spot signal did not remain fixed. With the present invention, as above described, these disadvantages are no longer present, and complete shading may be accomplished and -maintained in a simple and convenient manner.

Suppression or blanking of the white level signal by the generator 46 and subsequent blanking by the generator 88 are both employed since the white level signal suppression is so adjusted to give exact black level, i. e., zero signal level, so that changes in the volume or gain control setting will not alter the black level. The second blanking produced by the generator 88 produces a signal strength during the return time interval such that a fixed clipping margin may be assured.

As stated above, a conventional vertical shading signal cannot effectively be inserted at the same point in the circuit where the horizontal shading is inserted if the clamp circuit or automatic level setting circuit is used, since the operation of the clamp circuit will remove the vertical shading. Insertion of both shading signals at the same point is possible if the vertical shading signal is slotted in order that the signal intensity of the white signal level during blanking will not be affected, but such slottingis inconvenient to produce. According to the above described method of operation, with switches S--l .and *-2 in the clamp on position, the vertical shading is accomplished by cyclically varying the clamp level to compensate for variations in signal strength of successive line element-s due to the undesired vertical dark spot signal component. This is not the only manner in which vertical shading may be accomplished, and another method of producing vthe desired vertical shading will now be described.

Since the clamp circuit will effectively remove a vertical shading signal injected, rfor example, at the input of the amplifier 62, unless such shading is slotted, an effect similar to the slotting of the vertical shading may be accomplished by what would amount to a reverse situation in which the intensity of the white level signal impulses is varied to the exclusion of the intensity of the picture signals. This is accomplished by modulating the intensity of the Whitev level signal suppression impulses, the modulation occurring cyclically for each vertical deflection and the degree of modulation being determined in accordance with the required amount of vertical shading necessary to compensate for the undesired vertical dark spot signal. The set of curves shown in Figure 4 represents the operation of the present invention under these circumstances, in which case switches S--I and S-Z are positioned in a third position so that they will cooperate with lcontacts C13 and C23, respectively.

A series of picture signals will be generated by the television pickup tube similar to those represented by the curve B in Figure 4 when an image similar to that represented in Figure 6 is scanned and when a `dark spot -condition similar to that represented in Figure 5 is present. The effect of the dark spot signal will be apparent by an inspection of curve B of Figure 4. It will also be observed that the intensity of the white level signal is uniform, and this condition may be brought about by reason of the inclusion of the mask I9, as explained'above.

Furthermore, the produced dark spot signal is reduced to a minimum by proper adjustment of the position and intensity of the back-lighting source 20. 1 Y

In order to suppress the intens-ity of the White level signals and in order to inject a signal which will subsequently be effective toward the removal of the undesired ver-tical shading component, a voltage variation such as represented by the curve T in Figure 4 is injected in the anode circui-t of tube 3l). This signalA is` derived from the blanking signal generator 45 that normally supplies the signals for suppressing-the white level signal, except that instead of the signals having a uniform intensity as represented at 4B in Figure la, their intensity is cyclically modulated bythe desired vertical shading component which is supplied from the vertical shading signal generator |12 by Way of switch S-L These signals when applied to the blanking signal generator 46 are effective to lmodulate the intensity of the blanking or suppression signals so that their intensity will vary in a cyclic manner for each Vertical deflection of the cathode ray beam. When these signals (which occur at line deflection frequency) are combined with the picture signals present in the anode circuit of tube 30, a resultant wave form' will be produced similar to that represented by curve U in Figure 4. Here it maybe seen that the dark spot signal remains inthe signal series, but the intensity of the suppressed white level signal varies in a manner corresponding to the modulation oi the blanking or suppression impulses derived from the blanking signal generator 4t (see curve T of Figure 4). These signals are then subjected vto ampliiication by the video -or picture signal amplifier 42 and by the tube 50.

At the output of tube 5B (i. e., at the input of the ampliiier B2) a yvoltage variation will be present similar to that shown by curve U in Figure 4 except that the polarity will be reversed. At this point -in the circuit the horizontal shading signal'component is injected int-o the signal series. yas Was the case in the previously described conditions of operation, and the curves P of Figure 4 represent, for example, the necessary horizontal shading signal component. When the voltage variations represented by these' curves are combined lwith the signal represented by curve U of Figure 4, a resultant signal similar/to that represented by curve V of Figure 4 will be pron duced, and this signal is then supplied to the video or picture signal amplier 62. By an inspection of curve V, it -will 'be seen that all of the horizontaldark spot signal has been removed, but the average intensity of each successive line signal varies in a cyclic manner in accordance with the undesired vertical dark spot signal. The cyclic variation of the intensity of the signals occurring during the horizont-al beam return interval, however, is still present, this cyclic variation being a result `of the injection of thev modulated white level suppression signal in the anode circuit of tube Sli. It is seen, however, from curve V that the suppressed white pulse level corresponds now to the black or Zero level in each of the associated line signals.

After the signals are then amplied by the amplifier 62 and the amplier tu-be til, a Wave form such as that represented by curve V in Figure 4 is present in the anode circuit of tube 611i. Here the blanking impulses are inserted, as represented by curve F in Figure 4, and these impulses are derived from the blanking pulse generator 88. These impulses are of uniform intensity and their purpose, as explained above, is to .alter the poten-tial level of the signals during the beam return interval such that they correspond to an intensity exceeding black in the picture, this excess being inserted to depress ripples and noise signals below the black level so that they may be clipped off by tube 92. Theinjection of the blanking impulses then results in the formation of a signal such as that represented by the curve W of Figure 4.

Voltage variations such as represented by curve W are then supplied to amplifier tube 92 by way oi coupling condenser |86, since, as explained above, switch S-I and S'-2 are in contact with terminals C--l3 and C ZS, respectively. With switch `S-2 in this position, the clamp circuit is then effective but no modulation of the clamping level is employed and instead the anode of diode USE) and the cathodeV of diode H94 are maintained at a substantially uniform potential determined by the position of the movable Contact Ht along the potentiometer H2. Inasmuch as the clamp circuit is effective only during the beam return time interval, the signal applied to the control electrode of amplifier tube 92 will be clipped at a level indicated by the dotted line associated with curve W in Figure 4, which line, it will be observed, always lies a predetermined distance from the signal generated during the beam return time interval. This distance is equal to the height of the blanking pulses shown in curve F when a true black level is desired. This results in clip-ping the signal series at a uniform level corresponding to black in the picture, assuming, of course, that the position of the contact H6 is properly adjusted.

After the signal has been lclipped at this level, the result-ant signal present inthe anode circuit of tube 92 will then loe-similar to that represented byr curve K of Figure 4. By inspecting this curve it will be observed that not only is the horizontal dark spot signal removed, but the vertical dark spot signal has also been eliminated by action of the level setter or clamp circuit and the clipping action of tube 92, the clipping level being altered in accordance with variations in signal strength during theblanking or beam return time interval. Picture signals representative of black in the picture are, therefore, brought to 24` the same potential level and the intensity of the signals during the blanlnng interval has a value representative of black.

At this point in the circuit, i. e., in the anode circuit of tube 92, the synchronizing signals as represented by curve L of Figure 4 are inserted, and these when .combined with the signals repre'- sented by curve K produce a composite series of signals as shown by curvey N of Figure 4. This composite series of signals includes not only the picture signals, but also the blanking Signals and synchronizing signals, the latter extending in the direction of blacker-than-bl-ack and having an intensity greater than any picture signals. These signals, as explained above, may then be used directly to control the intensity of the cathode ray beam in a television picture tube, or they may be used to modul-ate a radio frequency carrier.

It may be seen, therefore, that the vertical shading component may` be introduced ahead of the level setting circuit, provided the vertical shading component is introduced as a variable intensity signal impulse effective only during the blanking interval as represented by the curve T in Figure 4. As explained above, if the vertical shading is introduced as a continuous voltage variation eiective not only during the blanking impulse time but also during the picture signal interval, this vertical shading component will remove the tilt of curve V in Figure 4 but will again be removed by the clamp circuit or level setting circuit. By, in effect, serratlng the vertical shading component so that this component airects the signal strength only during the blanking interval, Vertical shading may then effectively be introduced. Conversely, it isy also possible to similarly insert vertical shading by affecting or altering the intensity of the picture signals only at they output of tube 39,. for example, Without a'ecting the intensity, i, e., displacing the iconoscope blanking impulses or white level signals. This latter method of shading is not cntirely preferred, however, since the variable intensity shading impulses are then effective during the transmission of picture signals, and it is preferable not to inject any correction signal during this interval if the sameV correction may be accomplished by introducing a voltage which is eil'ective during times other than those intervals occupied by the picture signals, as, for eX- ample, during the blanking interval.

A still further and diierent method of compensating the undesired vertical dark spot may also be exercised by applying a control potential directly to the collector electrode of the television pickup or scanning tube. This method of vertical vshading will now be described, and in its description it will be assumed that the switches S-I and S-2 are in position to engage contacts C-ll and C-24, respectively.

It has been found that, as stated above, the generated' dark spot signal may be materially reduced in its inception by proper adjustment of the intensity and position `o1? the back-lighting source 2E. When the back-lighting source is properly adjusted, the amount of necessary vertical and horizontal shadingis reduced to a minirnium. A vertical shading correction may, therefore, be effected by modulating the potential of the ,collector electrode with respect to the mosaic electrode. When such a modulation potential is applied thereto, and when a maskv such as shown at I9 is used in conjunction with the television pickup tube, the Vertical shading-correction so introduced'wil-i then be sumcientt'o produce the desired results for virtually any lighting condition since no light from the object per se is permitted to strike the plhotosensitized walls of the television pickup tube. The diierential potential between the collector electrode and the mosaic electrode in a television transmitting tubeis normally relatively low. and is of the order oi a few volts. Any modulating potential that can be applied to the collector electrode must, therefore, be of relatively low potential if the television pickup tube is to continue to operate in a satisfactory manner.

In Figure la, the television pickup or camera tube I includes an electron gun structure which comprises a source of electrons or cathode, a control electrode and various accelerating electrodes. In normal practice, the second accelerating electrode is generally maintained at ground potential, whereas gun structure is maintained at a negative potential with respect to ground. The second accelerating electrode 240 in Figure la, instead of being directly connected to ground, is connected to ground by way of resistance 242 and condenser 244. The resistance 242 and condenser 244 are included in this connection in order that'the second accelerating relectrode 24|), which also functions as a collector electrode, may be potential modulated at vertical deflection frequency.

For accomplishing this purpose, vertical shadingk signals as derived from the vertical shading signal generator l12.are applied to the switch S-I by way of potentiometers |18 and |80. With the. switch S--I in positionC-.l4, these signals are then applied to the conductor 246 and in turn to a phase inverter 248. .The output from the phase inverter is then connected to the collector electrode or secondaccelerating electrode 24U ofr the television scanningV or pickup tube. The phase inverter 248 is necessary since the polarity o-r direction'of voltage change of the vertical shading signals as present at switch S-I is opposite to that which is required if vertical shading is to be accomplished by an application of rappropriate voltage'variations to the collector electrode ofr the television pickup tube. The phase inverter, therefore, amounts to no more than any simple orA convenient means for reversing the polarityor phase of the vertical shading signals.

Since, as stated above, the differential potential between tlhe collector electrode 240 andthe mosaic electrode l2 of the television pickup'tube l0 is quite low,l it is preferable that the modulating potentials as applied to the collector electrodenot exceed one volt maximum (peak-topeak). If the back-lighting source is properly positioned and regulated in intensity, the residual or remaining dark spot signal will be so small that a voltage of even considerable less than one volt will be sufcient to effect the desired vertical shading ,(i. e., the elimination of the vertical dark spot signal) in the television picture signal series. K 1

By modulating the potential ofthe collector electrode.' the efficiency of the television pickup tube may be affected, and since the potential applied to the Vcollector electrode at vertical deflection frequency, the eiliciency of the television pickup tube is caused to. -vary cyclically in the same manner and at the same rate. At the beginning of the vertical deflection cycle foreach individual field, Vthe efliciencyfof the television. pickup tube'is slightly decreased,

rthe cathode of the electronvaries ycyclically.

26 whereas at the end of each vertical deflection cycle the efciency of the television pickup rtube is increased.

The picture signals produced at the signal plate l2 of television pickup tube l will, therefore, be compensated for the vertical dark spot with the result that no vertical shading need be added at any succeeding point in the system. The horizontal shading or the variation in light response `in a horizontal direction has not at this point in the circuit been accomplished, with the result that horizontal shading signals must be injected at a later time.

The picture signals so generated, which are 4free of any undesired vertical dark spot signal, are then applied to the video signal pre-amplier 2b and to the succeeding amplifiers 30, 42, and d. At this point in the circuit the horizontal shading signal is injected in order to compensate for vthe horizontal dark spot signals so that the picture signals as applied to the amplier 62 are then free of any .dark spot signal.

No set of curves has been included in the drawings to represent the operation of the system under these conditions, since it is not believed that such would be necessary. The signal generated by the television pickup tube l0 with the collector electrode '24B modulated to compensate for the vertical dark spot would be similar tothatrepresented by curve B of Figure Li, except that the white level signal would have a constant amplitude a small vertica1 sawtooth component due to capacity coupling between the collector electrode 246 and' the mosaic electrode l2. This component would, howeven'be damped out. rIhis signal contains no vertical 4dark spot'component, and the curves sho-wn at P in Figure 4 represent the horizontal shading component injected at the cathode of tube 50. The wave form at the picture signal amplifier 62 of Figure 1b would, therefore, be substantially the same as curve V in Figure 4 except for a constant white pulse residual since the white level suppression impulses injected at the output of tube 30 are, in this case, of constant amplitude. At this point in the circuit both the horizontal and the vertical dark spot signals will have been removed by the above described means. The usual blanking pulse is then inserted at the anode of tube 64, and, as a result, a series of signals will be generated corresponding to curve W oi Figure 4 with the above exception.

Under conditions 'when the vertical dark spot signal is to be removed by a'slight modulation of the potential oi the collector electrode 240, the switches S-l and -S2 are positioned to'cooperate with contacts C--l4 and C--24, as stated above. With switch S-Z in position to contact terminal C--24, the black level setting is then under control of thek position of the movable contact H6 along thepotentiometer H2. Adjustment of this potentiometer determines the black level setting of the television system.

The circuit also includes the automatic level setting diodes |88, |90, 192 and |94. These diodes, `as described above, control automatically the black level, and since there is no modulation of the potential applied to the anode of diode and to the cathode of diode |94, the clipping level will be determined solely by the position of the movable contact along the potentiometer H2. Vertical shading signals under the presently assumed conditions are not applied to the clamp 'circuit lby way'of condenserv 232 and the signal may havesignals as the anode circuit of tube 92, a composite series `and different' dark spot 2"? since the switch "S-i .is in position to cooperate with `contact C id. .The automatic level .setting circuit, therefore, performs a clipping action,l as represented by the curve W in Figure Ll,

theV signa-lseries being clipped at a predetermined value above the peak black signals occurring during the 'horizontal -blanking interval. As

' described above, the automatic level setting circuitor clamp circuit is eifective only-during the Vinitial portion of each blanking interval.

' Accordingly, the signal present at the anode of tube 9?. will-be similar to that Arepresented by curve K in Fig re 4, andwhen the synchronizing supplied-by tube l22 are injected into of blanking, synchronizing and picture signals will then be produced similar to the curve shown at 4N- in Figure Ll. Y

By means of this last described ,method of operation,` it may now be appreciatedthat Vthe `vertical dark spot signal may be effectively elimi nated by modulating the. potential of the collector electrode l within predetermined limits. When the vertical dark spot signalis so removed,

vno further vertical `shading is necessary and for all purposes it may be assumed that the television pickup tube produces no undesired vertical dark spot signal.

Three separate methods have, therefore, been described for producing dark Ispot signal corre@ tion in a vertical direction, these being, respectively, the `modulation of the automatic level setting circuit orclamp circuit, the modulation of the `blanking or Y. suppression impulses applied tothe anode of tube titl, and the modulation of the potential of the collector electrodel-tll of` the television pickup tube, In each case, vthe horizontal shading component is injected in the sameimanner to compensate for the undesired horizontal Vdark spot signal, this shading signal being injected at the cathode of. tube Eil, for example. Inuorderto cause the sys-tem shown in Figures la andlb. to function in accordance with one or another of the three described methods of operation, it Ais merely necessary to appro priately adjust .the position of the switches S-,-i and E3- 2. i i

Irrespective of the particular methodY used for compensating for theundesired vertical dark spot signal, the dark spot signal may be reduced to a `minimum by proper adjustment of the position Aand intensity .of the backelighting. source 2Q.

Furthermore, by using the mask i9 to prevent lightfrom the image lfrom striking the. tube wall,

the necessary dark spot correction may be maintained substantially uniform, irrespective of picture light intensity since the light from the object` or picture to. be transmitted is projected only upon the mosaic electrode or target electrodel2 of the television image pickup or camera tube. Unless this is done, the amount and distribution of light which is permitted to fall upon the tube wall (which is inherently photcsensitive) will vary in accordance with the over-all light value of the subject being televised, and when such a variation takes` place, the intensity and wave shape of the dark spot signals vary correspondingly. If the mask i9y is not used and light is permitted to strike the inside of the tube wall, then a new setting of both the horizontal and vertical shading signal generators and their associated 4potentiometers is required for each change in the scenesince a new shading signal must be generated to compensate for the new signal. With the mask vundesired dark spot 28 I 9 positioned in the camera, the dark spot signal does not vary with picture light content, with the result that no alteration in the horizontaler vertical shading signal generators or their associated potentiometers is necessary, even though the light value of the subject being televised may change considerably as, for example, in outdoor pickups where the camera is moved to include more or less of the sky.

Although the present invention is described somewhat in detail, it is conceivable that various alterations and modications may be made therein Without departing from the spirit and scope thereof, and it is desired that any and all such alterations and modifications be considered with-- in the purview of the present invention.

Having now described my invention, what claim as new and desire to have protectedby Letters Patent is:

l. A television transmitting system including a television pickup tube of the electron storage type having a light responsive electrode on which an optical image may be projected, `means for scanning the light responsive electrode to pro duce image signals which inherently include un desired dark spot signals, means for amplifying the produced image and dark spot signals, an electron switching circuit, means for operating the electron switching circuit at line scanning rate for re-setting the zero level of the amplified signals to a predetermined average voltage level, means for cyclically altering the voltage level at held scanning rate, thereby to eliminate at least a portion of the undesired dark spot signals, and common means for varying the degree of amplification and for varying the degree of cyclic alteration of the .voltage level.

2. VA television transmitting system including a television pickup tube of the electron storage type having a light responsive `electrode on which a charge image isproduced in response to the projection thereon of an optical image, means for scanning the light responsive electrode to produce video signals which inherently include undesired dark spot signals, a capacity coupled amplifier for amplifying the produced video and dark spot signals, .a pair of shading signal generators for generating two series of shading control voltages, means for combining one series of shading control voltages withthe amplied signals'thereby to eliminate a portion of the undesired dark spot signals, an electron switching circuit responsive to impulses of line scanning frequency for re-setting the black signal level of the amplified signals to .a predetermined average voltage level, means responsive tothe other series of shading control voltages for cyclically altering the voltage level at eld scanning frequency, thereby completely to eliminate the remaining portion of the signals, and jointly operated controlmeans for varying the degree of ampliflcation and for varying the degree of cyclic alteration of the voltage level so that the undesired dark spot signals may be substantially completely removed irrespective of the degree of amplication of the signals.

' 3. A television transmitting system including a television pickup tube of the electron storage type having a light responsivemosaic electrode upon fying the produced image anddark spot signals,

an electron switchingv circuit responsive -tol impulses of horizontal scanning frequency for resetting the zero`level of the amplified signals to a predetermined average base voltage level, a vertical shading signal generator for generating compensating shading signals for eliminating the undesired vertical dark spot signals, and means for cyclically altering the base voltage level to which the amplified signals are re-set by the generated vertical shading signals therebyvto substantially eliminate the undesired vertical dark spot signals. v

4. A television transmitting system including a television pickup tube having Ya light responsive electrode on which an electrostatic charge image is produced in response tothe lprojection thereon of an optical image, means to repeatedly scan the target electrode by a cathode ray beam 'along a succession of substantially 4parallel lines in order to produce a succession of line image signals interspersed with white level signals that occur during the scanning beam return time interval, said image signals including undesired vertical dark spot signals, means .for .suppressing the interspersed white level signals byiapredetermined cyclically varying amount, the amountl determined in accordance with the extent of the undesired vertical dark spot'signal, means to amplify the line image signals and the fvariably suppressed white level signals, and means to shift the alternating current axis of the amplified signals in accordance with the intensity of the variably suppressed white level signals to thereby effectively remove the undesired vertical dark spot signal.

5. A television transmitting system including a television camera tube having a light responsive electrode on which an electrostatic charge image is produced in response to the projection thereon of an optical image, means to scan the target electrode by a cathode ray beam in bi-dimensional directions at different rates in order to produce a succession of line image signals interspersed with white level signals that occur during the scanning beam return time interval, said line image signals including undesired vertical dark spot signals, means for suppressing the interspersed white level signals by a predetermined cyclically varying amount, the amount determined in accordance with the extent of the undesired vertical dark spot signal, means to amplify the line image signals and the variably suppressed white level signals, a further amplifying tube including a control electrode, means to apply the amplified signals to said control electrode, a source of biasing potential, an electron switching circuit connected between said control electrode and said source of biasing potential, and means for rendering said switching circuit eective to connect said control electrode to said source of biasing potential during the variably suppressed white level signal intervals so that the alternating current axis of the amplified signals may be shifted in accordance with the intensity of the variably suppressed White level signals to thereby eiectively remove the undesired vertical dark spot signal.

6. A television transmitting system including a television camera tube of the electron storage type having a light responsive electrode on which an electrostatic charge image is produced in response to the projection thereon of an optical image, means to scan the target electrode by a cathode ray beam in bi-dimensional directions at different rates in orderl to produce a'. succession' of line image signal-sinterspersed with white level signals that occur during the scanning beam return time interval, said line image signals including undesired horizontal and vertical dark spot signals, means for suppressing the interspersed white level signals by av predetermined cyclically varying amount, the amount determined in accordancewith theextent of the undesired vertical dark spot signal, means to amplify the line image signals and the variably suppressed white level signals, a horizontal shading signal generator for producing voltage variations in accordance with the intensity of the undesired horizontal dark spot signal, means to combine the horizontal shadingv signal with the amplied line picture signals, a further amplifying tube including a control electrode, means signals to said control electrode, a source of biasing potential, an electron switching circuit connected between said control electrode and said source of biasing potential, and means for rendering said switching circ 't eifective tol connect said control Velectrode to said source of biasing potential during the suppressed white level signal intervals so that the alternating current axis of the amplified signals may be shifted in accordance with the intensity of the variably suppressed white level signals to thereby eifectively remove the undesired vertical dark spot signal whereby the undesired vertical and horizontal dark spot signals may both be removed from the produced image signals.

7. A television transmitting system including a television pickup tube having a light responsive target electrode upon which an electrostatic charge image is produced when an optical image is projected thereon, means to scan the target electrode by a'eathode ray beam to produce a succession of line image signals interspersed with white level signals corresponding to the scanning beam return time interval, said image signals also including undesired vertical dark spot signals, a vertical shading signal generator for producing voltage variations to compensate for the undesired vertical clark spot signals, means for suppressing the interspersed white level signals, the degree of suppression being cyclically varied in accordance with the generated vertical shading signals, means for amplifying the line image signal-s and the variably suppressed white level signals, and means for re-setting the black signal level of the amplified line picture signals to a predetermined voltage level in accordance with the intensity of the variably suppressed white level signal to thereby effectively remove the undesired vertical dark spot signal.

8. A television transmitting system including a television pickup tube having a light responsive target electrode upon which an electrostatic charge image is produced when an optical image is projected thereon, means to scan the target electrode by a cathode ray beam in bidimensional directions at diiferent rates to produce a succession of video signal-s interspersed with white level signals corresponding to the scanning beam return time interval, said video signals also including undesired horizontal and vertical dark spot signals, a. vertical shading signal generator for producing voltage variations to compensate for the undesired kvertical dark spot signals, a horizontal shading signal generator for producing voltage variations to compensate for the undesired horizontal dark spot signals, means for supto apply the amplifiedv 

